Semiconductor power converter circuits using fuses for over-current protection of the semiconductor switching elements are known.
In a first example, the carrying capacity of the semiconductor elements is chosen to be greater than the fusing current, so that when an overcurrent or a shortcircuit occurs, the corresponding fuse or fuses are fused, interrupting the current, protecting the semiconductor elements. In a second example, found for instance in power converter circuits comprising plural semiconductor elements connected in parallel with one another, a fuse is connected in series with each semiconductor element. When one of the semiconductor elements is damaged, the corresponding fuse is fused, separating the damaged semiconductor element from other normal semiconductor elements. This allow continuous operation of the apparatus without interruption. Otherwise when a current above the semiconductor's current carrying capacity flows, the semiconductor element may explode, causing arcing. In some cases this causes a serious accident or a fire. In order to prevent explosion of the semiconductor elements, fuses are used to rapidly limit the shortcircuit current.
A further problem is experienced in connection with protection of large current, high voltage power converters. In fact it has been impossible to employ fuses for circuit protection of power converter apparatus of higher voltage, for the following reasons. As the current capacity of the power converter apparatus increases, the short-circuit current that flows through the semiconductor elements increases likewise. As an example, the DC voltage in a thyristor converter for use in DC power transmission can reach as high as 250 kV. Single unit fuses that can withstand such high voltages are not available. A plurality of lower voltage rating fuses can be connected in series; however, the diversity of fusing characteristics of the plural fuses will tend to result in a lack of uniformity in the arc voltages which exist when the respective fuses are fused. Accordingly, the remaining fuses are exposed to voltage beyond their inherent capabilities, and failures result.
Therefore, in thyristor converters of higher voltage, overcurrent protection has been provided relying on the gate control of thyristors, i.e., the gate shift or gate block thereof, instead of fuses. Effective overcurrent protection of thyristor converters of 3 kA rated current for DC power transmission has been achieved using circuits of this type. However, this current value is inadequate for power converter apparatus for use in connection with nuclear fusion experiments; such devices have DC power requirements on the order of several tens kV and several hundreds kA. Thyristor base control is inadequate for this application.
The increased demands for larger currents at higher voltages to be provided by power converter equipment therefore require improvement of the fused protection arrangements described above.